The present invention is concerned with a tandem master cylinder for use with hydraulically operated vehicular brakes. There is at least one master cylinder piston including a piston restoring spring fixed by a central clamping element and a valve body displaceable relative to said master cylinder piston. The valve body opens a compensating bore on said master cylinder piston. The clamping element for the piston restoring spring on its end facing said master cyinder piston carries the valve body which, in the resting position, is a small distance ahead of said compensating bore in the front wall of said master cylinder piston. The clamping element holds the valve body in the at rest position of said master cylinder piston within the bore opening.
In hydraulically operated vehicular brakes, provision has to be made for the amount of hydraulic fluid contained in the pressure chamber of said master cylinder so that the entire brake system can be automatically refilled from the supply reservoir, if so required. In that respect, it is generally known to provide in the cylinder wall, directly ahead of the master cylinder piston, in the resting position thereof, a compensating bore via which the pressure chamber of the master cylinder, in the resting position, is in communication with the supply reservoir. This will safeguard the supply of brake fluid; moreover, it will permit equalization of pressure between the brake system and the supply reservoir, e.g. in case of an expansion of the brake fluid in the brake system when heated. Upon actuation of the master cylinder piston, the piston closes the compensating bore at the beginning of the piston stroke so that the pressure build-up required for a braking operation can take place in the brake system.
The port of said compensating bore into the cylinder interior produces an interruption of the smooth cylinder running surface over which the piston seal will have to move upon each stroke of the master cylinder piston. This may cause a strain on the piston seal.
To avoid an interruption of the cylinder running surface swept over by the piston seal, it has previously been attempted to replace the compensating bore provided in the cylinder wall, with a compensating port in the master cylinder piston, that is closed by a valve body upon actuation of the master cylinder piston.
In such a prior art construction (FR-PS No. 1 571 663), the master cylinder piston includes a central compensating bore. The valve body closing the said compensating bore is formed at the front end of a piston actuating rod which, relative to the master cylinder piston, is axially displaceable by a small distance. The small but unavoidable distance produces undesirable play in the actuation of the brake. The restoring spring required for the retraction movement of the master cylinder piston does not engage the master cylinder piston as such but rather the piston actuating rod. It is not positive that the compensating bore upon relief of the piston be released in due course thus causing the risk that at least a temporary vacuum will arise in the brake likely to result in a penetration of air into the master cylinder.